1. Field of the Invention
The present invention relates to a mobile X-ray device of the type having an X-ray system including an X-ray source and a planar X-radiation detector, the X-ray system being displaceable relative to the subject for picking up series of 2D projections of a subject for a reconstruction of at least one 3D image of the subject. Further, the invention relates to a method for determining projection geometries for such an X-ray device.
2. Description of the Prior Art
X-ray devices of the aforementioned type are used, for example, in the medical field to reconstruct 3D images of a body part from a series of picked up 2D projections of the body part of a patient. The projection geometries, i.e., the positions of the X-ray source and of the X-ray detector, as well as the projection angle with respect to each of the individual 2D projections of the series, must be known in order to reconstruct accurate 3D images from the 2D projections that are picked up by the X-ray system.
U.S. Pat. No. 4,053,780 discloses an axially operating, room mounted tomographic scanning device with an X-ray system that has an X-ray source and an X-ray detector, this X-ray system being attached to a frame and being displaceable relative to the subject for picking up projections of the subject. Further, the scanning device has means for displacing the X-ray system, which means, in a calibration process, allow the determination of data required for purposes of reconstructing an image.
German PS 35 31 741 describes a tomography device with a movable system for generating radiation and with a movable radiation measuring system that can be set up independently of the position of the radiation generating system.
U.S. Pat. No. 5,014,293 describes an X-ray computed tomography device with a device cart and with an X-ray system, which is arranged at the C-arm of the device cart. The X-ray system has an X-ray source that emits a fan-shaped X-ray bundle and an X-ray detector in the form of a detector array. For acquiring X-ray pickups, the C-arm can be moved around a subject, whereby the fan-shaped X-ray bundle penetrates the subject an strikes on the detector array. 3D images of the subject can be acquired from the X-ray pickups.
German PS 43 35 300 describes a computer tomography device with an X-ray system and a measuring means for the position of the center of rotation, which measuring means controls a focus deviation unit in the sense of a guidance of the focus given a displacement of the center of rotation.
German OS 197 46 093 discloses a movable X-ray device of the type initially described, wherein transmission and reception devices for sound waves or electromagnetic waves are provided, which allow the detection of projection geometries during the pickup of a series of 2D projections.
German PS 195 12 819 describes an X-ray computed tomography device, wherein, during the pickup of 2D projections, X-ray-positive markers are arranged in the measuring field above and below the area of the subject to be examined. The markers are imaged in the 2D projections. The projection geometries can be detected for each 2D projection by evaluating the 2D projections.
In the last two cited known X-ray devices in particular, online detection of the projection geometries is used, namely detection of the projection geometries during the pickup of a series of 2D projections of a subject, or at least during the recording of data during the pickup of a series of 2D projections, which allow the detection of the projection geometries. Such online detection is necessary in these devices, since the X-ray devices are mechanically unstable with respect to the displacement movement of the X-ray system because, being mobile, a lighter structure must be used (as opposed to the heavier, and thus more stable, structure which can be used in a non-mobile device). These mechanical instabilities rule out an exactly reproducible displacement movement of the X-ray system, so that the positions of the X-ray source and the X-ray receiver, at the respective point in time of the pickup of a 2D projection by means of, for example, locators or position transmitters that are present in the X-ray devices, cannot be exactly determined such that 3D images of high quality could be reconstructed.
Online detection of the projection geometries, however, requires the use of a large amount of computing power in order to, in a desired way, arrive at 3D images of the subject in an optimally short time after the series of 2D projections have been picked up. Therefore, realtime reconstruction of 3D images is only possible when expensive computers with high computing power are utilized.
An object of the present invention is to provide the projection geometries, which are necessary for reconstructing 3D images, in a simplified way for a movable X-ray device.
This object is inventively achieved in a mobile X-ray device with an X-ray system that includes an X-ray source and a planar X-ray detector, the X-ray system being displaceable relative to the subject for picking up a series of 2D projections of a subject for reconstruction of at least one 3D image of the subject and which has a displacement arrangement for assuring a reproducible displacement movement of the X-ray system by means of which the projection geometries of the X-ray system required for the reconstruction of a 3D image are determined in a calibration process prior to the pickup of series of 2D projections of the subject. As a result of the displacement arrangement which assures a reproducible displacement movement of the X-ray system, it is possible to determine the projection geometries prior to the pickup of series of 2D projections of a subject in a (normally) onetime calibration process. Thus, the once detected projection geometries can be repeatedly used for reconstructing 3D images from series of 2D projections of different subjects picked up at different points in time, which hitherto has been regarded as impossible with respect to mobile X-ray devices due to the light construction necessary for mobility and the instabilities associated therewith. The realization of an offline determination of the projection geometries, namely a determination of the projection geometries prior to the actual pickup of 2D projections of a subject, is based on the recognition that the twists of the C-armxe2x80x94which occur when the X-ray system arranged at the C-arm is displacedxe2x80x94can be considered as mechanical constants; these twists leading to deviations of the X-ray system from its ideal displacement movement, which can be detected by means of locators. Under equal conditions guaranteed by the drive for the displacement of the C-arm, this assumption proves sufficiently correct, so that the displacement movement of the C-arm or X-ray system can be regarded as reproducible.
In an embodiment of the invention the displacement arrangement has a digitally controlled drive, which effects the displacement movement of the X-ray system. The digitally controlled drive preferably is a software-controlled drive, and can be a pulse motor according to an embodiment of the invention, and makes a precise displacement of the X-ray system possible. The pulse motor cooperates, for example, with the C-arm, which accepts the X-ray system. Thus, individual positions can be repeatedly reached in an accurate manner within 500 xcexcxc2x0 in the displacement movement of the X-ray system, so that the positions of the X-ray source and the X-ray detector identified during the calibration can be almost exactly achieved again with respect to later displacement processes of the X-ray system. Thus, the conditions that are present for later measurements with regard to the mechanical constants of the X-ray system correspond to the conditions that are present during the calibration.
According to a version of the invention, the X-ray system is arranged at a carrying device borne by a support which is displaceable along an axis in a holding device and which is pivotably arranged for rotation around the axis relative to the holding device, and the displacement arrangement include compensation components, which exhibit a defined effective direction and which compensate any mechanical play that is present between the support and the holding device.
In a further version of the invention the holding device, in a height-adjustable manner, is arranged at a lifting device having at least two elements that can be moved relative to one another, these elements forming additional compensation components, which exhibit a defined effective direction and which compensate any mechanical play that is present between the elements of the lifting device.
The compensation components can be locking components, for example, such as springs or spring-biased locking pins, which, during the calibration process and during later measuring processes, clamp the support against the holding device, or clamp, the elements of the lifting device against one another, in a defined effective direction, so that equal conditions always exist with respect to different measuring processes.
According to an embodiment of the invention, the carrying device for the X-ray system is a C-arm.
The above object also is achieved in an inventive method for the offline determination of the projection geometries for a movable X-ray device, wherein the X-ray system of the X-ray device is arranged at a carrying device, which is held in a support and which is displaceable relative to the support, and wherein the support can be pivoted relative to a holding device around an axis extending through the support and the holding device and/or wherein the support, along the axis, is displaceably arranged at the holding device and/or wherein the holding device is height-adjustably arranged at a lifting device. The inventive method includes the following steps:
a) adjusting a first pivot position of the support relative to the holding device and/or a first position of the support relative to the holding device along the axis and/or a first height adjustment of the holding device,
b) arranging a phantom, which is provided for determining the projection geometries, relative to the X-ray system such that it can be penetrated by an X-ray bundle proceeding from the X-ray source to the X-ray detector,
c) picking up a series of 2D projections of the phantom by displacing the carrying device in the support,
d) evaluating the 2D projections of the phantom for detecting (identifying) the projection geometries for each of the 2D projections,
e) storing the projection geometries for the selected position of the support and/or the holding device, and
f) repeating the steps a) through e) if and when the position of the support and/or the holding device changes.
The inventive method makes it possible, in a simple way, to detect projection geometries prior to an examination of a subject for all desired displacement movements or adjustments of the X-ray system relative to a subject and makes it possible to have these projection geometries available for later subject measurements, or for the reconstruction of 3D images from measured 2D projections of a subject. Normally, as far as the construction of the X-ray device does not change, the calibration must be undertaken only once. Therefore, a computerized online determination of projection geometries for reconstructing 3D images from a series of 2D projections is no longer necessary.